1. Field of the Invention
The present invention generally relates to optical projection systems, and more particularly to a condenser lens for an overhead projector which is constructed to balance overall brightness and degree of uniformity.
2. Description of the Prior Art
Overhead projectors (OHP's) are known in the art, and generally comprise a base having a light source, a stage area, and a projector head located above the stage. A condenser lens in the base directs light toward the stage, and a Fresnel lens at the stage gathers the light and directs it to a projection lens in the head, which projects any transparent image placed on the stage. Two important characteristics of the projected image are overall brightness and uniformity of the illumination.
Brightness has become more important as the use of color transparencies and liquid crystal display (LCD) panels in combination with OHP's has become more prevalent. Three important factors in achieving high brightness are lamp construction, collection of the light and utilization of the light which has been collected.
Lamp construction can be varied in several ways to obtain increased brightness. One variable is the means by which electrical energy is converted into light. The two most common types of projection lamps according to this classification scheme are incandescent and arc discharge lamps. Arc lamps convert the electricity to light more efficiently than incandescent lamps do, but their cost and the cost of the appropriate power supply has restricted their use to the most expensive projectors. Incandescent lamps are used in the vast majority of projectors. In particular, tungsten-halogen incandescent lamps are used because their light output is relatively constant over their life. If one is restricted to tungsten-halogen technology, the only way to increase brightness without sacrificing lamp life is to use a higher wattage lamp.
The inefficiency of incandescent lamps is due to the fact that most of the electrical energy is converted into infrared radiation. Infrared radiation is absorbed by many materials, including glass, causing them to heat up. The amount of heat generated by a normal projection lamp is not enough to soften a glass condenser, but it does cause the condenser to expand in proportion to the amount of energy received. This means that the central portion of the condenser, which is closest to the filament and receives the most radiation, expands more than the outer portions. Such differential expansion can cause a condenser to crack. For lower wattage incandescent projection lamps, tempering a soda-lime glass condenser is sufficient to protect it from cracking. Higher wattage lamps require a type of glass which is more resistant to thermal stress, such as borosilicate glasses. PYREX brand glass is a common glass of this type.
Another technique for increasing brightness is the judicious selection of a condenser which gathers as much of the light from the source as possible and directs it through the stage. The amount of light collected by the condenser is governed by its collection angle. This is defined as the angle between the axis of rotation of the condenser and a line from the center of the light source to the edge of the condenser. This angle is affected by the distance from the filament to the closest side of the condenser and the diameter of the condenser. The condenser is normally placed as close to the lamp as is mechanically feasible, so only the diameter is variable. Increasing the diameter increases the collection angle, but it also increases the necessary thickness of the condenser. Increasing the thickness increases both its cost and its susceptibility to thermal stress.
Third of the aforementioned contributors to brightness is the utilization of the collected light. If all of the collected light could be aimed so that it would pass through the stage, utilization would be perfect. Due to several factors, however, increased utilization often results in poor distribution of the light across the stage and, hence, uneven illumination of the projection screen. In the simplest example of this relationship, a system may be constructed wherein nearly all of the light passes through the stage in a circular beam but, due to the square shape of the stage, the corners are dark or black. The beam may be expanded and flattened toward a more square shape, but there will still be some fading of illumination at the corners, and increasingly more light will fall outside the stage perimeter as the uniformity of illumination improves, thus decreasing overall brightness. This trade-off is discussed in U.S. Pat. Nos. 1,946,088, 2,637,242, 5,010,465 and 5,092,672.
Some evenness in light distribution may be achieved by providing an optical system having several lens components. This multiplicity of lenses, however, is contrary to the desideratum of minimizing part count to simplify manufacturability and reduce cost, and multiple lenses also decreases brightness due to surface reflections. An alternative approach is to vary one or more parameters in the OHP optical system other than the condensing system. For example, a common prior art technique for "optimizing" the brightness/uniformity balance has been to adjust the distance from the lamp filament to the Fresnel lens at the stage while viewing the projection screen (using the same condenser throughout the process). Another trial-and-error approach is to substitute different condenser lenses from a selection of such lenses. The most obvious flaw in these approaches is the subjective judgment involved in determining the optimum balance of brightness and uniformity. These are also ad hoc approaches which may suffer from unnecessary design constraints, such as the specific lens constructions in the limited selection of lenses. It would, therefore, be desirable and advantageous to devise a method for determining the construction of a condenser lens providing improved brightness/uniformity balance, the construction being limited to a single lens component, and based on other known parameters of the OHP's optical system.